Thermoplastic polymer blends have found wide use in various fields such as car parts, appliance parts, hand-held utensils and other goods where a combination of durability and processability are valued. As used herein, “blend” shall mean a combination of two or more discrete components that may or may not be readily separable after combination, and the term “thermoplastic polymer blends” includes, without limitation, thermoplastic polyolefins, thermoplastic elastomers and thermoplastic vulcanizates. Thermoplastic polymer blends often are composed of a discrete phase of non-thermoplastic polymer dispersed in a matrix of thermoplastic polymer. The non-thermoplastic polymer phase is added to provide physical characteristics not present in the thermoplastic polymer absent the additional phase. Additionally, if the non-thermoplastic polymer phase is composed of material with limited processability, dispersing the non-thermoplastic polymer phase in a matrix of thermoplastic polymer imparts at least some of the processability characteristics of thermoplastic polymers to the blends.
Thermoplastic elastomers (“TPEs”) are a special class of thermoplastic polymer blends and have a combination of both thermoplastic and elastic properties. Defined generally, a thermoplastic elastomer is a polymer or blend of polymers that can be processed and recycled in the same way as a conventional thermoplastic, yet has at least some of the properties and performance of a vulcanized rubber at service temperature. Blends or alloys of plastic and elastomeric rubber have become increasingly important in the production of higher performance thermoplastic elastomers, particularly for the replacement of thermoset rubber in various applications.
TPEs which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic polymer with an elastomeric composition in a way such that the rubber is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. Early work with vulcanized composition is found in U.S. Pat. No. 3,037,954, which discloses static vulcanization as well as the technique of dynamic vulcanization (explained further below). The resulting composition is a microgel dispersion of cured elastomer, such as EPDM rubber, butyl rubber, chlorinated butyl rubber, polybutadiene or polyisoprene in an uncured matrix of thermoplastic polymer such as polypropylene.
Depending on the ultimate application, such TPE compositions may comprise one or a mixture of thermoplastic materials such as propylene homo- or co-polymers, and like thermoplastics used in combination with one or a mixture of cured or non-cured elastomers such as ethylene propylene rubber (“EPM”), ethylene propylene diene rubber (“EPDM”), diolefin rubber, butyl rubber or similar elastomers. TPE compositions may also be prepared where the thermoplastic material used also includes an engineering resin having good high temperature properties, such as a polyamide or polyester using in combination with a cured or non-cured elastomer. Examples of such TPE compositions and methods of processing such compositions, including methods of dynamic vulcanization, may be found in U.S. Pat. Nos. 4,130,534, 4,130,535, 4,594,390, 5,177,147, and 5,290,886 and W/O 92/02582, which are incorporated by reference as if fully included herein.
Olefinic thermoplastic elastomers (thermoplastic polyolefins, or “TPOs”) are produced from an olefinic thermoplastic and a natural or synthetic rubber. Dynamically vulcanized thermoplastic elastomers (thermoplastic vulcanizates, or “TPVs”), a special subset of TPEs, also have a combination of thermoplastic and elastic properties. TPVs are prepared by melt mixing and shearing at least one each of a thermoplastic polymer, a vulcanizable elastomer and a curing agent. The vulcanizable elastomer is dynamically cured during the shearing and mixing and is intimately and uniformly dispersed as a particulate phase within a continuous phase of the thermoplastic polymer. See, for example U.S. Pat. Nos. 4,311,628 and 6,147,160, which are incorporated by reference as if fully included herein.
TPE compositions are normally melt processed using conventional thermoplastic molding equipment such as by injection molding, compression molding, extrusion, blow molding or other thermoforming techniques. In such TPE compositions, the presence of the rubber component does not necessarily improve the processability of the composition. In fact, where the rubber component in partially or fully cured (or cross-linked) in situ during the mixing of the TPE components (known as “dynamic vulcanization”), or where a dynamically vulcanized TPE composition is further processed, there are heavier demands placed upon processing machinery as compared with the processing of a thermoplastic composition which is free of cured elastomer.
Often TPEs, including polypropylene-based TPEs, suffer from long cycle times in thermoforming applications. “Cycle time” for thermoforming applications may generally be described as the duration from the introduction of molten polymer into a mold to the release of the molded part from the mold. A long cycle time may be the result of many factors, including low crystallization temperatures and low crystallization rates in the thermoplastic phase. Long cycle times lead to inefficiencies in the thermoforming process, increasing costs and decreasing productivity. It is known to alter the crystallization kinetics of thermoplastics, particularly propylene-based thermoplastics using additives such as nucleating agents. While not wishing to be bound by theory, it is believed that nucleating agents form nucleation centers, or active centers, on which formation of a polymer crystal may start. For slow crystallizing polymers like polypropylene, nucleating agents often will result in a clearer, more rapidly crystallizing polymer than will exist absent the agent. In thermoforming processes, the higher crystallization temperature or rate induced by the nucleating agent will reduce cycle times (as a major component of cycle time is the time required to cool the formed polymeric article to a point where it can be ejected from the die without losing its shape). Additionally, a TPE based on a nucleated thermoplastic may exhibit a variety of other improved properties, such as stiffness.
However, the use of nucleating agents with TPVs has been problematic at best. Nucleation agents are known to (1) provide little or no effect on the TPV when used in conventional quantities, (2) interfere with the rubber component curing process and (3) cause unwanted weight gain in the TPV. Why nucleation agents cause these problems has, heretofore, been unknown, thus the use of nucleation agents with TPVs has not generally been successful in altering the characteristics of TPVs to increase crystallization kinetics and reduce thermoforming cycle time.
It would be desirable to have a TPE (or TPV) with superior characteristics to reduce or eliminate known deficiencies of traditional TPEs used in thermoforming processes. It would likewise be desirable to have a process to produce a TPV with superior characteristics that may be more efficiently used in thermoforming processes.